Re: [Plib-users] Problem with rotations

[Ben W. <za...@ec...>]

That is a good question, and I don't know the answer. But why would it work
for header and role.

h=1
p=0
r=0
this works fine and rotates around. same with
h=0
p=0
r=1
it also works fine. Its just
h=0
p=1
r=0
That causing the problem. And why would it jump to another axe and the come
back.
I did also try this with just a matrix, were I created the rotation matrix
with sgMakeRot4, then muliply it by the location and running the transform
without converting it back to euler, but I got the same result.
The thing I find weird is that its only when it hits one axe that it jumps
to anyother one.
Anyway I don't know, I can't find the problem, and I have a test problem
that does it, but I just don't understand why its doing it, so I can't
really explain.
Ben


----- Original Message -----
From: "Steve Baker" <sjb...@ai...>
To: <pli...@li...>
Sent: Tuesday, September 10, 2002 1:47 AM
Subject: Re: [Plib-users] Problem with rotations


>
>
> Ben Woodhead wrote:
>
> > Ya, I don't have a problem with that.. It seems to be sgMakeRotMat(mat,
> > h,p,r) where pitch is set to something other then 0. Then I converted it
> > back to sgCoord and used transform...
>
> Could your confusion be this:
>
>     * You take three angles (H,P,R) and convert them into a matrix M
>       (using sgMakeRotMat...or whatever).
>
>     * Then you convert M *BACK* into an sgCoord and extract (H',P',R').
>
> Do you expect (or rely upon) H' == H,  P' == P and R' == R ???
>
> If so, then that's not right.  There are an infinite number of values
> for H,P,R that all generate the exact same rotation matrix.  Hence, when
> you convert a matrix *BACK* into three Euler angles, you don't get the
> same three angles back...although the three angles you get *DO* perform
> the same effective rotation as the three you put in.
>
> Hence, it's possible to say things like:
>
>      H =   0
>      P = 180
>      R =   0
>
> ...then convert to a matrix...convert back again and get back:
>
>      H' = 180
>      P' =   0
>      R' = 180
>
> ...totally different numbers - but having the same exact same effect.
>
> If that is your problem, then it's not something that we can fix - it's
> just the way math works.
>
> ----------------------------- Steve Baker -------------------------------
> Mail : <sjb...@ai...>   WorkMail: <sj...@li...>
> URLs : http://www.sjbaker.org
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